DATAJPA-1787 - Updates Specifications documentation.

Updates Specifications documentation to use java 8 lambdas instead of anonymous classes.
This improves readability and reduces boilerplate code.

Original pull request: #429.

src/main/asciidoc/jpa.adoc

@@ -809,31 +809,23 @@ Specifications can easily be used to build an extensible set of predicates on to
 ----
 public class CustomerSpecs {
 
-  public static Specification<Customer> isLongTermCustomer() {
-    return new Specification<Customer>() {
-      public Predicate toPredicate(Root<Customer> root, CriteriaQuery<?> query,
-            CriteriaBuilder builder) {
 
-         LocalDate date = new LocalDate().minusYears(2);
-         return builder.lessThan(root.get(Customer_.createdAt), date);
-      }
+  public static Specification<Customer> isLongTermCustomer() {
+    return (root, query, builder) -> {
+      LocalDate date = LocalDate.now().minusYears(2);
+      return builder.lessThan(root.get(Customer_.createdAt), date);
     };
   }
 
   public static Specification<Customer> hasSalesOfMoreThan(MonetaryAmount value) {
-    return new Specification<Customer>() {
-      public Predicate toPredicate(Root<T> root, CriteriaQuery<?> query,
-            CriteriaBuilder builder) {
-
-         // build query here
-      }
+    return (root, query, builder) -> {
+      // build query here
     };
   }
 }
 ----
 ====
 
-Admittedly, the amount of boilerplate leaves room for improvement (that may eventually be reduced by Java 8 closures), but the client side becomes much nicer, as you will see later in this section.
 The `Customer_` type is a metamodel type generated using the JPA Metamodel generator (see the link:$$https://docs.jboss.org/hibernate/jpamodelgen/1.0/reference/en-US/html_single/#whatisit$$[Hibernate implementation's documentation for an example]).
 So the expression, `Customer_.createdAt`, assumes the `Customer` has a `createdAt` attribute of type `Date`.
 Besides that, we have expressed some criteria on a business requirement abstraction level and created executable `Specifications`.